To assess the clinical efficacy of allogeneic umbilical cord mesenchymal stem cell transplantation in the treatment of patients with severe type 1 diabetes, defined as with the history of diabetic ketoacidosis from diagnosis. All the patients are intravenously administrated with single-dose or double-dose umbilical cord mesenchymal stem cell and followed up for 36 months. Clinical and laboratory manifestations are compared before and after transplantation.